Arterial Blood Gas – Why don’t we do them more often?
Jennifer Waldrop, DVM, DACVECC
Myth 1: It’s too hard to obtain the sample.
Arterial blood gases may seem hard to acquire but actually are no harder in medium to large dogs than sampling a vein. Because the artery has a thicker muscular wall, it is can be slightly more difficult to pierce, but by using your finger to anchor the artery, you can make it easier. The most common place to try is the dorsal pedal artery on the medial aspect of the metatarsus. Another common location is the femoral artery or the lingual artery if under anesthesia. The femoral artery must be held off manually for 5 minutes after sampling. When you have a large dog under anesthesia, use this time to practice feeling an artery and even trying to obtain a sample in more controlled circumstances.
Myth 2: It’s too hard to handle the sample.
Arterial samples are handled the same way as venous with a few exceptions. Do not agitate an arterial sample as it can falsely elevate your oxygen content. You do not need to purchase a specific arterial sampling syringe, although those are nice. Aspirate heparin into a tuberculin then evacuate it; this will nicely receive the sample and prevent clot formation. The needle can be pierced into the rubber stopper of a lab tube to prevent further oxygenation. You have about 10 minutes to analyze the sample at room temperature, but if you need longer, the sample can be on ice for 30 minutes without degradation.
Myth 3: I need a special machine to analyze it.
I-STAT cartridges and some other in-house blood analyzers are designed to analyze oxygen and carbon dioxide concentrations. Check with the manufacturer if you are not sure, but any machine that measures pH and bicarbonate is likely to be able to analyze PaO2 and PaCO2.
Why bother if I have a pulse oximeter? Pulse oximeters are wonderful machines but have limitations. Sometimes they have difficulty picking up a signal in animals with pigmented mucosa, icterus or if the patient is not perfusing well. Measuring saturation of oxygen (SaO2) with a pulse oximeter only evaluates oxygenation and not ventilation. This can lead to false assumptions that animals with normal oxygenation do not have lung disease. Many patients can hyperventilate to tolerable SaO2 values but may collapse due to exhaustion if they have to keep hyperventilating.
Myth 4: It’s too hard to interpret arterial blood gases.
With minimal practice, interpreting the basics of oxygenation and ventilation values is not difficult, i.e. determining that the patient is within normal limits or not. The two main values of importance are PaO2 (oxygenation) and PaCO2 (ventilation). The normal value for PaO2 on room air is 80-100 mmHg and for PaCO2, 35-45 mmHg. Most of us will want to perform an arterial blood gas to determine if a patient is hypoxic and needing supplemental oxygen. That would be verified by an arterial blood gas in the following two scenarios:
Scenario 1: The PaO2 is less than 80 mm Hg. If you want to verify this finding with a pulse oximeter, you would see a SaO2 of less than 95%.
Scenario 2: The PaO2 is in the 80s but the PaCO2 is less than 30 mmHg. Finding this result indicates that the patient is hyperventilating significantly to normalize their oxygen level.
Both scenarios document a need for oxygen supplementation.
More information about arterial blood gas interpretation can be found in many books and on VIN. You can also take labs locally and at national conferences to practice sampling and interpretation. We at BluePearl are also available to help in any way. Please let us know if you would like more information or if we can help if you have a difficult respiratory case.